In the preceding funding period we have identified recurrent mutations and copy number increases of KIT in 30-40% of melanomas arising on chronically sun-damaged skin (CSD), acral sites. Emerging clinical evidence indicates that these genetic alterations are a strong indicator for clinical responses to already approved KIT inhibitors. By contrast, KIT alterations are absent in melanoma arising on the non-CSD skin, which frequently harbor BRAF mutations, identifying them as biologically distinct. Differences in mutation spectra, chromosomal aberrations, and clinical and histopathological presentation implicate the KIT pathway as an important contributor in acral, mucosal, and CSD melanoma types. Despite these advances, the oncogenic alterations in the majority of these melanomas are currently not known as they infrequently harbor BRAF mutations. Preliminary studies implicate additional genes downstream of KIT as candidate genes, including the scaffolding protein GAB2, the protein phosphatase SHP2, amplification of the two p70S6 kinase genes as additional targets for somatic activation. We hypothesize that together these findings indicate a wider role for KIT pathway activation in these melanoma types, which would have therapeutic and diagnostic implications. We will validate the contribution of genetic alterations in the KIT pathway to the malignant phenotype and response to therapy and aim to discover additional alterations. Our studies will involve comprehensive genomic characterization of archival tissues to identify genetic alterations and determine in which combination they occur (aim 1), functional analyses in vitro models representing the most common of the findings in human tumors to characterize their effect on sensitivity and resistance to targeted agents (aim 2), and tumor samples from patients with KIT alterations who were treated with KIT inhibitors, to determine factors of primary and secondary resistance to treatment in a clinical setting (aim 3).